Rechargeable batteries of the kind described above are used as power supplies for a variety of portable electric apparatuses, such as mobile telephones, portable computers, personal digital assistants (PDA), hand-held communicators, etc. A mobile telephone for any commonly used mobile telecommunication system (for instance GSM) will be used throughout this document as an example of such portable electric apparatuses, thereby specifically not limiting the present invention to mobile telephones only.
Rechargeable batteries of the type described above are provided with more or less sophisticated safety circuits for preventing an occurring malfunction from creating damage of a more serious kind, for instance due to fire or explosion. A frequently used type of batteries are batteries comprising lithium-based battery cells. Lithium-based batteries have several advantages, such as a high energy density and thereby a long operation time between subsequent rechargings, but such batteries are sensible to abnormally high voltages or currents as a consequence of an occurred electric malfunction, for instance a short circuit. These days rechargeable batteries are often provided with a safety circuit in the form of a printed circuit board in the battery pack. A circuit is arranged on this printed circuit board for monitoring various battery parameters, such as the cell voltage across individual battery cells, the current through the cells, etc. Furthermore, the circuit board comprises active and resetable circuits, which in response to the detected battery parameters are arranged to for instance conduct away excessively strong electric currents when a malfunction condition occurs.
Such a rather complex safety arrangement has a disadvantage particularly as regards the relatively high component and manufacturing costs. In addition the circuit board has a certain physical shape, which makes it difficult to minimize the size of the battery pack to such an extent, which otherwise would be desired.
A less complex solution is disclosed in JP-A-2250634, wherein the safety circuit comprises a reference voltage device in the form of a comparator, which is arranged to detect an occurring overvoltage situation and in response thereof supply a voltage indicative of the overvoltage situation to an active device in the form of a thyristor. As a consequence, the thyristor will switch from a normal non-conductive state to a conductive state, wherein a shortcircuiting current will flow through the thyristor and through a fuse device, which will release or melt and cause an open circuit protecting the battery from the over-voltage.
A similar solution is disclosed in JP-A-7154922, where a transistor acts as the active device and a microcomputer acts as the reference voltage device.
U.S. Pat. No. 5,164,874 relates to an apparatus for protecting against overvoltage, comprising a PN junction zener diode as the reference voltage device and a PNPN junction thyristor as the active device. The reference voltage device and the active device are integrally formed as an integrated circuit (IC) with two terminals, which are arranged to be electrically connected to a first and a second side, respectively, of a load to be protected. A conventional fuse device is separately connected in series with the protective IC apparatus.